Material Differences

Additive manufacturing demands different thinking about the metal stock.

Adopting additive manufacturing entails even more than a shift in thinking about how to make the part. There is also different thinking about material. The success of an additive build can turn on material considerations so subtle as to be, well, granular.

John Hunter has a perspective on this. He is the general manager of the newly opened U.S. offce of LPW Technology, a company focused on metal powder for additive manufacturing. Founded in the U.K., LPW works with additive machine makers to tailor powders to their equipment, and also supplies powder to those machines’ end users. I spoke with Hunter at the new facility, which is in Pittsburgh. He says those end-user customers typically are companies that have moved past the most basic level with additive production.

Starting out, he says, shops adopting additive manufacturing buy powder from the machine OEM. That source can provide an alloy formulation along with proven machine parameters for it. But later, the shop might become comfortable enough at fine-tuning parameters to try comparable stock from a third party, or try a novel formulation.

LPW has experience with all of the major metal additive machines, Hunter says. The firm counsels customers on which particle size distribution (PSD), for example, works better for a particular machine type. Anecdotes illustrate some other material considerations that can determine success, including:

Purity. Hunter says one customer came to LPW because aluminum alloy parts were cracking. Both powder and parameters were supplied by the OEM—the build should have been successful. Micrographs revealed the culprits. Just a half-dozen particles of Inconel 625 were the impurities from which the cracks propagated. The customer was relieved to have this answer: It was not cleaning its machine thoroughly enough between jobs.

Flow. In additive manufacturing, the powder metal has to move. LPW once tested powder from four sources that was all identical in terms of measured specs, but still not identical in performance. At a given energy, only one sample delivered a fully dense part. Thus, some different spec was needed to predict performance. The answer proved to be powder fow rate. Like pharmaceutical frms, LPW now uses rheometers to gauge this parameter.

Exposure. The metal powder changes over time, Hunter notes. Some changes can be forestalled; his company developed new packaging when previous containers proved too porous to humidity. But other changes are unavoidable, since the additive builds change the composition. Smaller grains are consumed in the frst few builds, so the PSD changes with use. That variation is a material consideration unlike anything that a machine shop, for example, previously had to consider.

In 2014, LPW introduced a service called PowderSolve to address this problem. Users enter key measurements of their own powder samples into online software to track material changes and predict performance. Instead of guessing when a batch of material has reached its limits, the aim is for users to be able to determine this for certain. That is, the aim is to save the user from having to stop at using just 70 percent of the material, but to go on instead to use perhaps 90 percent of it. In fact, that same knowledge could also reveal where powder that has passed the limits for one application might still be useful for an application with different requirements.